Build zones and no build zones.

Like an area where you have tools and can build with in a perimeter.

Spawn a special entity at the coordinates you want and then use the cantool function (http://wiki.garrysmod.com/?title=Gamemode.CanTool) and check if that player is a certain distance from your special entity and return false if they are to far.

I’m unsure as to why you would want to limit someone from using a tool though.

Can you do that in reverse where you can spawn an entity and allow you to build?